In recent years, the natural environment surrounding the earth has significantly deteriorated. Above all, in thermal power plants and the like located around the world, SO2 and soot and dust in flue gases generated as a result of combustion of fossil fuels are one of the main causes of environmental problems such as air pollution, and it has become mainstream to install a wet flue-gas desulfurization system for treatment of the flue gases.
Particularly recently, reductions in the concentration of emission values of SO2 and soot and dust in flue gases have been demanded, while the inlet SO2 concentration has increased due to diversification of boiler fuels and the like, and thus there is a pressing need to develop a high-performance flue-gas desulfurization system.
An example of a flue-gas desulfurization system of a conventional art is shown in FIG. 4 and FIG. 5. The flue-gas desulfurization system is constructed mainly with an absorber shell 1, a gas inlet port 2, a gas outlet port 3, an absorbent liquid spray section 4, a spray nozzle 5, a recirculation pump 6, a recirculation tank section 7, an oxidation agitator 8, a posterior air pipe 10, a propeller 11, a gypsum slurry bleed pipe 20, a gypsum dewatering system 21, and a mist eliminator section 30.
A flue gas G from a boiler is introduced through the gas inlet port 2 and makes gas-liquid contact with an absorbent liquid sprayed from the spray nozzle 5 of the absorbent liquid spray section 4 to thereby become a clean gas, and is emitted through the gas outlet port 3 after accompanying mist is removed therefrom by the mist eliminator section 30.
Moreover, the absorbent liquid brought in gas-liquid contact falls in the absorber shell 1 and is stored into the recirculation tank section 7. In the recirculation tank section 7, air 9 to be fed is atomized into a large amount of fine bubbles by the propeller 11 that rotates in conjunction with the oxidation agitator 8, and oxygen in the air dissolves in the absorbent liquid. In the recirculation tank section 7, calcium sulfite is produced by a neutralization reaction between absorbed SO2 and calcium carbonate that is fed to the recirculation tank section 7 by an unillustrated calcium carbonate feeding system, and the calcium sulfite is oxidized by oxygen dissolved in the absorbent liquid to produce gypsum. The absorbent liquid in the recirculation tank section 7 where gypsum exists as slurry is sent again to the spray nozzle 5 by the recirculation pump 6, is partly sent to the gypsum dewatering system 21 through the gypsum slurry bleed pipe 20, and is therein separated into solid gypsum and water.
In the abovementioned conventional art, the oxidation air 9 is fed from the posterior air pipe 10 with a feed port, which is at the rear of the propeller 11 of the agitator 8, to the recirculation tank section 7 as shown in FIG. 5. This is a mode of increasing the air utilization rate by atomizing, by a shearing force generated by a rotation of the propeller 11, the oxidation air 9 fed to the recirculation tank section 7 into a large amount of fine bubbles to thereby increase a gas-liquid contact area with the absorbent liquid (Japanese Published Unexamined Patent Application No. 2001-120946).
Moreover, there is a method for feeding air to the front of the agitator propeller 11 (Japanese Published Unexamined Patent Application No. H08-949, Japanese Published Unexamined Patent Application No. 2000-317259). In this method, employed is a mode of making air accompany a discharge flow of the absorbent liquid resulting from a rotation of the propeller 11 to thereby uniformly disperse air bubbles in the recirculation tank section while atomizing the same to fine bubbles.